This chapter of book deals with the problem of estimating the value of the active power of the ac signal in the presence of subharmonics and interharmonics. The method can also be applied analogously for determining the RMS value of basic electrical values. The analysis makes use of the most general model of the voltage and current signal, i.e. the most complex spectral content that can be expected to appear in practice. A simulation of the suggested procedure has been done as well, as a final confirmation of the possibility to have accurate processing by using the synchronized sampling technique, even in the presence of subharmonics and interharmonics.